A computer system is generally comprised of several component parts including a processor, computer memory, a data bus, and other peripheral devices and components. The processor accesses, modifies, and writes data to random access memory. The data contained in computer memory is transferred to the processor through the data bus. Memory is a semiconductor storage device for holding programs or data. There're three basic forms of memories which include dynamic storage in which data must be constantly refreshed and in which data are erased when power is no longer applied to the cell, non-volatile memory in which data remains permanently in the cell even when power is not applied, and static memory which does not need to be refreshed but which loses its data when power is no longer applied to the cell. In a computer system, non-volatile memory is used for long-term storage of programs and data which seldom or never changes, and volatile memory devices are used for the short-term storage of program instructions and data during the execution of a program. According to the application functions, memories can be categorized into read only memory (ROM) and random access memory (RAM). As the name implies, the read only memory is only read accessible. A ROM device cannot be rewritten once it has been programmed. Embedded software applications use ROM to store embedded code and data records. The processor in an embedded software application retrieves each instruction from ROM and executes it. The random access memory can perform both write and read operations. Random access memory (RAM) also differs from ROM in that when power is disconnected from RAM, the data stored in random access memory is lost whereas when power is disconnected from ROM the data stored in read only memory remains. The read only memory is further categorized into a mask read only memory programmable read only memory (PROM), erasable programmable read only memory (EPROM), and electrically erasable programmable read only memory (EEPROM). Whereas, the random access memory can be further categorized into a static random access memory (SRAM) and a dynamic random access memory (DRAM). Static read/write random-access memory (SRAM) is a type of volatile memory in which the data, once it is written to a memory location, remains stored there as long as power is applied to the memory chip. Dynamic random access memories (DRAMs) are a mainstay in the semiconductor industry. DRAMs are data storage devices that store data as charge on a storage capacitor. In DRAM, the data stored at each location is periodically refreshed by reading it and then writing it back again to the same location, or else it disappears. One type of DRAM transfers information synchronously with a clock signal. This type of DRAM is referred to as synchronous DRAM (SDRAM). SDRAM transfers information once every clock cycle of the clock signal, such as the rising edge of the clock signal. Nevertheless, DDR DRAM transfers data on each edge of the clock signal, thus doubling the peak throughput of the memory device as compared with SDRAM. The magnetoresistive random access memory (MRAM) is an alternative memory device to dynamic random access memory (DRAM). An MRAM device uses magnetic orientations to retain data in its memory cells. MRAM devices are relatively fast, are nonvolatile, consume relatively little power, and do not suffer from a write cycle limitation. Programmable read only memory (PROM) allows the device manufacturer to program the embedded code. This allows for revisions in the code but still does not allow for modification or erasure of the ROM once it has been programmed. Erasable programmable read only memory (EPROM), electronically erasable programmable read only memory (EEPROM) and flash memory are a growing class of non-volatile storage integrated circuits based on floating gate transistors. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, and cellular phones.
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